1. Field of the Invention
The present invention relates to a polybutylene naphthalate-based resin composition used as insulating material. In particular, it relates to a polybutylene naphthalate-based resin composition with excellent heat resistance, flame retardancy, abrasion resistance, and hydrolysis resistance, and an electric cable using the polybutylene naphthalate-based resin composition.
2. Description of the Related Art
Conventionally, polyvinyl chloride resin (PVC) is used as typical electrical insulating material. This PVC insulating material is excellent in having high utility and being inexpensive, but its waste disposal causes environmental pollution, e.g., its incineration after disposal produces chlorine-containing gas. Accordingly, in recent years, there is a demand for insulating material other than PVC.
Also, in transportation fields such as vehicles, trains, etc., with reducing vehicle body weight and saving wiring space for energy saving, there are demands for reduction in weight and thickness of electric cables.
Applying the conventional PVC material for reduction in weight and thickness of electric cables fails to achieve flame retardancy or abrasion resistance required.
On the other hand, among polyester resins which are engineering plastic polymers, polybutylene terephthalate (PBT) is a crystalline polymer, and is excellent in heat resistance, mechanical strength, gas barrier, chemical resistance, abrasion resistance, low solubility, and moldability, and is therefore used in vehicle fuel tubes, liquid crystal glass abrader members, semiconductor-related members, etc. (see JP-A-2005-281465, JP-A-2006-152122, and JP-A-2007-45952 listed below).
Because of having the above features, these engineering plastics are expected to be able to achieve reduction in weight and thickness of electric cables.
Refer to JP-A-2005-281465, JP-A-2006-152122, JP-A-2007-45952, JP-A-2006-111655, JP-A-2006-111873, JP-A-2005-213441, JP-A-2004-193117, and JP-A-2002-358837, for example.
However, the polyester resins, which are a crystalline polymer, have the problem of variation in crystallinity in a producing process or under a specified environment. In particular, heat treatment causes the crystallization to progress, and there is therefore a fear that the tensile elongation property, which is important to insulating material for electric cables, will deteriorate.
JP-A-2006-111655 and JP-A-2006-111873 listed above report that heat treatment or crystallization accelerant addition enhances crystallinity to enhance mechanical strength, high-speed moldability and productivity. However, accelerating crystallization is thought to cause deterioration of the elongation property.
Also, JP-A-2005-213441 listed above discloses that crystallization progression can be retarded by introducing a flexible monomer as polyester-resin raw material, but it does not disclose any elongation property. Further, JP-A-2004-193117 finds out that adding to a polyester resin a resin containing a functional group to react with polyester-based resins inhibits crazing and inhibits a decrease in insulation breakdown voltage and allows excellent high-temperature insulation property, but it does not mention any elongation property with heat treatment of electric cable insulating material.
Further, JP-A-2002-358837 listed above suggests a polyester resin composition for flat cables and sheathes, which contains a thermoplastic aromatic polyester, a specified polyester block copolymer, an olefin-acrylic ester copolymer modified with a glycidyl compound, and optionally a phosphorus-based flame retardant. However, although the phosphorus-based flame retardant used in the polyester resin composition is non-halogen, the polyester resin composition is not suitable for market demands for non-phosphorus-based flame retardants.